I got a new HD5850 gpu, and it bombed out in every single game even when you're pc's just idling it would've bombed out as well.
Turned out, it was overheating, got it replaced with a brand new XFX gamer's edt, and everything went smooth sailing from thre onward.
Maybe get Joker to check out your gpu temps?
I'm also playing on mine, but I just have the normal macbook (the old white one)
It's got a dual core processor, 2Gb ram and nvidia gt330M gpu
But the game 's laggy, I don't know whether it's the laptop itself or whether it's my internet connection (using iburst atm).
Details set to low running at 1280x800 res
If you press Ctr + R its shows your FPS, that should give you an idea of if its the machine or ur connection. I've gotten the odd day where it will be laggy as hell but that seems to be more the connection than anything else.
Have you tried bootcamp on it yet?